The present invention relates to a process for preparing a synthetic hydrogel and more particularly to a process for preparing a hydrogel from a polyvinyl alcohol which hydrogel has superior characteristics not observed in conventional natural or synthetic hydrogels.
Hydrogels (hydrous gels) are considered to be very promising materials because they have high water content, a high substance permeability and are soft to the feel. But their use is extremely limited because they have a serious drawback such that they are inferior in mechanical strength. There have been proposed various hardening means (strength improving means) wherein hydrogels (or gelling components) inferior in mechanical strength are treated with formaldehyde, glutaraldehyde, terephthalaldehyde or hexamethylenediamine. But these chemical treatments use reagents which arh harmful to organism, so the use of the treated hydrogels would cause various troubles.
Moreover, the aforesaid chemical treatments usually cause a great deterioration of the superior characteristic (high water content) of hydrogels, so it is impossible to expect much from such treatments. Therefore, the irradiation method is expected as the only method for hardening weak hydrogels [see N. A. Peppas et al., J. Biomed. Mater. Res., 4, 423 (1977) and H. Singh et al., J. Sci. Ind. Res., 39, 162 (March 1980)].
However, the irradiation method not only requires a special equipment but also its effect is not so remarkable, and therefore, in general, its practical application is difficult. By the application of radiation, moreover, the superior characteristic of hydrogels is often lost (or deteriorated).
Furthermore, since a high molecular weight compound embedded (entrapped) in the interior of a hydrogel is gradually released to the exterior of the hydrogel, hydrogels are taken note of also as a release control (slowly releasing) material for medicines, agricultural chemicals, fertilizers, perfumes and fishing baits. For example, various attempts are well known, such as the embedding of dibucaine (a local anesthetic) and sulfamethizole into agar and devil's tongue jelly [see Masahiro Nakano, Kagaku to Kogyo, 32, 569 (1979) and Maku (Membrane), 3 (6) 386 (1978)], the embedding of aromatic components into carrageenan, etc. [see CMC Technical Report No. 6, p.184 (1980)], the embedding of pilocarpine (a glaucoma treating medicine) into poly(ethylene - vinyl acetate) or collagen [see U.S. Pat. No. 3,618,604 (1971) and A.L. Rubin et al., J. Clin. Pharmacol., 13, 309 (1973)], and the embedding of fluorouracil (a carcinostatic agent) into poly(2-hydroxyethylmethacrylate) [see M. Arlen et al., Arch. Surg., 105, 100 (1972)]. However, not to mention agar and carrageenan, it goes without saying that also with respect to many other hydrogels, their weakness are often pointed out and cause troubles in practical use.
Hydrogels are hydrous and have a high permeability for low molecular weight substances. Besides, they can contain (capture) giant molecules or their aggregate. Therefore, hydrogels are expected also as an embedding (immobilizing) material for active carbon or physiologically active substances such as enzymes. There are also well known the embedding of active carbon into gelatin, poly(2-hydroxyethylmethacrylate) (adsorption type artificial kidney) [see J. D. Andrade et al., Trans. Am. Soc. Art. Int. Organs, 18, 473 (1972); B. G. Gazzard et al., The Lancet, 29, 1301 (1974); Nobuo Nakabayashi, Kobunshi Ronbunsyu, 34, (4) 317, 323 (1977)], the embedding of enzyme into collagen (enzyme membrane, enzyme electrode) [see Karube and Suzuki, Kagaku Kogaku, 40, 139 (1976)]. However, because the gels are weak or fragile, it has often been pointed out that the gels are cracked or the embedded matters leak out.
The present invention is the first to provide a method of obtaining hydrogels superior in mechanical strength without using any of these chemicals or radiation.
The present invention uses a polyvinyl alcohol as a starting material for the preparation of a hydrogel. As to the method of gelling a polyvinyl alcohol (hydrogel preparing method), there have already been proposed many methods. But, as will be summarized below, all of these methods involve problems in point of operation or in the properties of product.
(1) By air-drying an aqueous polyvinyl alcohol solution there is obtained a wet or dry film, which, however, is a mere weak film inferior in water-resisting property and having no integrity in water and is used merely in limited applications (see Japanese Patent Publication No.9523/1965).
(2) Also by adding an acid into an aqueous suspension containing water soluble polymers and tetraethyl silicate to produce composite sol then followed by air-drying, there merely is obtained the same film as in the above (1). In this connection, it has also been proposed to add an acid into the aqueous suspension to produce composite sol followed by freeze-drying. But the resultant film rather deteriorates in its strength and is scarcely moldable (see Japanese Patent Puhlications Nos.30358/1980 and 11311/1980).
(3) The gelling method involving application of cobalt 60 (.gamma.-ray) to an aqueous polyvinyl alcohol solution is well known. In this case, however, not only a special equipment (irradiation equipment) is essential and the irradiation cost is high, but also the resultant gel is weak and often requires an additional hardening means (secondary hardening treatment). Therefore, the gel obtained by this method is difficult to be utilized except in special applications wherein a highly viscous liquid (or a soft gel) is desired such as an artificial vitreous body (intra-eyeball filling liquid) (see J. Material Sci., 1974, 1815 and Japanese Patent Laid Open No. 55647/1975).
(4) Also, it has long been well known that an aqueous polyvinyl alcohol solution gels upon mixing with boric acid (or an aqueous boric acid solution) or with borax (or an aqueous borax solution) (Note: borax=sodium tetraborate decahydrate). However, the resultant gel is weak and has fluidity; besides, it is torn immediately when picked up with finger tips, so it is difficult to retain its shape after molding [see J. Am. Chem. Sci., 60, 1045 (1938) and French Pat. No. 743942 (1933)].
Moreover, although this borax gel can exist in an alkaline condition, it collapses easily at a pH value not more than 8. Therefore, this borax gel is difficult to be utilized except in special applications.
(5) It is also well known that gel is formed by adding a very small amount (0.1-0.2 wt. %) of polyvinyl alcohol to kaolin (kaolinite) or bentonite. On the basis of this principle, an attempt to modify the surface soil of a stadium into a soil from which dust is relatively difficult to rise by scattering a polyvinyl alcohol (a dilute aqueous solution) over the said surface soil, an attempt to improve the water permeability or water retaining property of the soil of fields by scattering a small amount of polyvinyl alcohol (a dilute aqueous solution) over the fields, and the technique of promoting the flocculation and precipitation of clay (colloidal particles) in a muddy water by adding a small amount of polyvinyl alcohol into the muddy water, are also well known. However, the gels formed in these cases are very fragile (difficult to distinguish from a mere soil in external appearance) and collapse very easily even in the state of dry powder, not to mention in water [see J. Agr. Sci., 47, 117 (1956)].
(6) There have also been proposed various methods of gelling a polyvinyl alcohol using phenols such as phenol, naphthol and Congo Red or amino compounds or metallic compounds such as titanium, chromium and zirconium compounds. In all of these methods, however, the same drawbacks as in the foregoing (4) are encountered [see Nippon Kagaku Zasshi, 72, 1058 (1951) and Japanese Patent Publication No.23204/1965].
(7) It is also well known to gel a polyvinyl alcohol using cross-linking agents or copolymer components such as aldehydes, dialdehydes, unsaturated nitriles, diisocyanates, trimethylolmelamine, epichlorohydrin, bis-(.beta.-hydroxyethyl)sulfone, polyacrylic acid, dimethylolurea and maleic anhydride. In this case, however, not only a procedure using chemical reagents is needed, but also it is difficult to obtain a strong gel of a high water content [see Textile Res. J., (3), 189 (1962) and British Pat. No.742,900 (1958)].
(8) Also, it has long been well known to gel an aqueous polyvinyl alcohol solution by allowing it to stand at a low temperature not higher than 40.degree. C., particularly not higher than 5.degree. to 18.degree. C. [see Kominami et al., Kobunshi Kagaku, 12, 218 (1955), Maeda et al., Kobunshi Kagaku, 13, 193 (1956), and Kogyo Kagaku Zasshi, 59, 809 (1956)].
However, gels formed at room temperature or thereabout are fragile like agar and carrageenan. Besides, they are dissolved on stirring merely vigorously or with water added or on warming a little [see Kominami et al., Kobunshi Kagaku, 12, 218 (1955), and Takahashi and Sakurada, Kobunshi Kagaku, 13, 502 (1956)].
It is also well known that low temperatures are preferable to obtain a cooled gel of an aqueous polyvinyl alcohol solution. For example, there is known an example in which the cooled gel is formed at 18.degree. C. or even at 0.degree. C. or lower [see Maeda et al., Kobunshi Kagaku, 13, 193 (1956), Japanese Patent Publication No.12854/1972, and Takahashi et al., Polymer J., 6, 103 (1974)].
However, the gels thereby obtained are weak gels (or viscous liquids) like agar, carrageenan or jelly and are very sticky. In addition, those gels are inferior in water-resisting property. Under water, they swell to a remarkable extent and soften, a part of which is dissolved out into water and the remainder becomes paste-like. Furthermore, under water or in a warm water at 40.degree.-50.degree. C., those gels rapidly get out of shape and disperse and dissolve in water. Because of these drawbacks, their use are extremely restricted inevitably.
(9) It is also known to add a small amount of polyvinyl alcohol into aqueous solutions of water-soluble high polymers having a gelling capacility such as agarose, albumin, alginate, curdlan, carrageenan, casein, CMC (sodium carboxymethyl cellulose), furcellaran, gelatin, methyl cellulose, pectin, starch, tamarind gum, xanthan gum, tragacanth gum and guar gum, and then let cool the resulting solutions, or immerse the solutions into a gelling agent-containing bath (coagulation bath), or freeze-dry the solutions [see Fragrance Journal (Japan) 2, (7) 68 (1974) and Japanese Patent Publication Nos.25210/1981 and 25211/1981]. However, even by such a method there merely is obtained a weak, less water-resistive, viscous liquid or non-fluid gel. or a water-soluble, dry powder (freeze-dried powder).